Two stage optical magnification and image correction system

ABSTRACT

A display having a two-stage optical process is disclosed. This two-stage system enables a relatively compact and inexpensive display. A display screen projects an image that passes through a first lens or lens system. The collimated light is then diffused by one of several methods so as to increase the ultimate viewer&#39;s eyebox. The diffused image then undergoes a magnification process involving total internal reflection within a second lens. The light exiting the second lens is magnified to the level desired at a low cost and a small size.

[0001] This application is a continuation-in-part of application Ser.No. 09/589,836, filed Jun. 8, 2000 for TWO STAGE OPTICAL SYSTEM FOR HEADMOUNTED DISPLAY.

FIELD OF THE INVENTION

[0002] The present invention relates to an optical display system thatmay be used with head mounted or hand held display systems. Moreparticularly, the invention relates to a two-stage optical system forthis display system that comprises a first stage for magnification andimage sizing and a second stage that includes a total internalreflection process.

BACKGROUND OF THE INVENTION

[0003] A real image refers to an image that is observed directly by theunaided human eye. A photograph is an example of a real image.Electronic displays that provide a real image generally provide someform of display surface on which the real image is formed and viewed. Areal image can be observed by the unaided eye when a viewing surface ispositioned at its location. Examples of electronic displays that providereal images include liquid crystal displays, CRT monitors, andprojection screens.

[0004] In contrast with real images, a virtual image is an image thatappears to be coming from a location where no real image exists. Bydefinition, a virtual image can exist at a location where no displaysurface exists. The size of the virtual image therefore is not limitedby the size of a display surface. An example of a virtual image is theimage of fine print viewed through a magnifying glass. The print notonly appears larger, but it also appears to be located substantiallybehind the surface where the print actually exists. Virtual imagedisplays thus have the advantage of eliminating the need for a largedisplay surface in order to provide a large image to the viewer.

[0005] Software and computer hardware for creating virtual images haveimproved steadily over time. However, generating sizable displays isexpensive and greatly increases the cost of display devices. In order toprovide a viewer with as complete a virtual reality experience aspossible, the image he sees should fill his field of vision. The viewermust also be able to look around at his environment. In order toaccomplish these goals, displays need to provide a virtual image to theviewer as opposed to a real image.

[0006] A virtual display must initially form a source image that is thenrendered by an optical system to create the virtual image. A substantialadvantage of a virtual electronic display is that the source imageinitially created may be as small as can be usefully reimaged by theoptical system. As a result, virtual electronic display systems mayeffectively use very small displays to form the source image. Pixelsizes may be as small as a few microns in diameter. At this size, theunaided eye cannot resolve images. Rather, in order to view the sourceimage formed by the display, substantial magnification of the opticalsystem is required.

[0007] A virtual image must be created by an optical system of somekind. In a real image electronic display, it is the eye and the viewingsurface properties that determine the viewing parameters. By contrast,in a virtual image display, the optical system determines most of theviewing parameters.

[0008] There are three important parameters relating to the ease ofviewing the image associated with virtual image display systems. Thefirst parameter is the eye relief. This refers to the maximum distancefrom the eye which the optical system can be held and have the eye stillsee the entire virtual image. Optical devices which provide a eye reliefwhich is a short distance from the optic are undesirable due to theinconvenience and discomfort associated with placing the eye in closeproximity with the optic. It is therefore preferred that an opticprovide a long eye relief in order to enable the magnified image to beviewed through the optic at a comfortable and convenient range ofdistances from the optic.

[0009] The second parameter relating to the ease of viewing a virtualimage is the apparent angular width of the virtual image, commonlyreferred to as the field of view of the virtual image. The full field ofview is defined as the ratio of the largest apparent dimension of thevirtual image to the apparent distance to the virtual image. It isgenerally equivalent to the field of view for a real image displaysurface.

[0010] The third parameter relating to the ease of viewing a virtualimage is the transverse distance that the eye may move with respect tothe optical system and still have the eye see the entire virtual imagethrough the optical system. This is commonly referred to as the“eyebox.” The size of the eyebox is determined by the eye relief andsize of the exit pupil of the display system. The exit pupil is theplace where the eye must be placed in order to see the whole image atonce. A large exit pupil has been found to be one of the most importantfactors in determining viewing comfort by nearly all users we havetested. A large exit pupil and eyebox will also accommodate the range ofpupil motion needed as the eyeballs rotate to scan through the viewingangle of the image, as well as to accommodate the variation in theinterpupillary distance among the user population.

[0011] A need currently exists for an inexpensive, compact, virtualimage display system that can be used in a hand held or head mountedapparatus that provides a virtual image that is positionable within asmall volume and has a large exit pupil and eye relief and uses smalldisplays.

[0012] It is recognized that one of the primary factors driving up thecost of virtual reality display systems is the cost of the initialdisplay. Prior art display systems have been created that use smalldisplays coupled to magnification systems. These generate the largervirtual images the viewer sees. However, prior art magnificationprocesses are bulky and can make head mounted or handheld displaysystems unwieldy and cumbersome. Also, these display systems do not workas well with handheld devices.

[0013] Therefore, the need exists for a lightweight display system thatoperably presents a visual display occupying all or almost all of theviewer's field of vision to a wearer that is both comfortable andrelatively inexpensive.

[0014] Further, video images are recorded with various aspect ratios,e.g., 4:3 and 16:9. The display screen showing the unmagnified videoimage will necessarily have fixed dimensions. Therefore, for all imageswith aspect ratios that do not match the fixed dimensions of the displayscreen, there will be distortion in either the height or the width ofthe image dependent upon the relation between the aspect ratio of theimage and that of the display. It would be an improvement if the imagecould be adjusted so that it appears undistorted to a viewer.

SUMMARY OF THE INVENTION

[0015] An object of the invention is to create a display system that islightweight, convenient, and relatively inexpensive.

[0016] This and other objects of the invention are accomplished by adisplay system having a two-stage optical system where the second stagemagnification is accomplished using total internal reflectiontechniques. This two-stage system is usable in relatively compact andinexpensive display systems. In a preferred embodiment, the head-mounteddisplay system has two sections extending rearward around the sides ofthe head. Within each section, a display screen projects an image thatpasses through a first lens that adjusts the size of the image. It canmagnify the image, alter the aspect ratio of the image, or both. Theimage then undergoes total internal reflection within a subsequent lens,resulting in magnification of the image. The viewer is presented with avirtual image many times larger than the original display.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a schematic illustration of a first preferred embodimentof the two-stage process;

[0018]FIG. 2 is a schematic illustration of a second preferredembodiment of the two-stage process;

[0019]FIG. 3 is an illustration of a lens used for total internalreflection;

[0020]FIG. 4 is a schematic illustration of a TIR lens incorporating afolding mirror integrally along the leading edge;

[0021]FIG. 5 is an illustration of a third preferred embodiment of thetwo-stage process using a projection element between the first andsecond stages of the process;

[0022]FIG. 6 is an illustration of a fourth preferred embodiment of thetwo-stage process;

[0023]FIG. 7 is an illustration of a fifth preferred embodiment of thetwo-stage process;

[0024]FIG. 8A shows a plot of transmission as a function of the angle ofincidence for light passing from air into a TIR lens;

[0025]FIG. 8B shows a plot of transmission as a function of the angle ofincidence for light passing from a TIR lens into air.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026]FIG. 1 is an illustration of the magnification process used forthis invention. It includes a display 30, a first magnifier 32 and atotally internally reflecting (TIR) lens 34. Light rays emitted by agenerated image can be seen traveling through the system and ultimatelybeing received by a viewer's eyeball 40.

[0027] The image to be magnified originates on the display screen 30,which will generally have a screen size of 0.5″ diagonally. Astechnology improves smaller and smaller screen displays may be used withthis invention. Displays with 0.5″ diagonals or less are much cheaperthan 1″ or 2″ displays. There are three basic types of displays: (1)self-illuminating or emissive, such as OLEDs or FEDs, (2) back lit ortransmissive, such as AMLCDs, or (3) front lit or reflective, such asLCOS displays.

[0028] The screen used in our preferred embodiment is a transmissivedisplay. A first stage optical system 32 is placed in the path of thelight being projected from the display. We have primarily usedtransmissive displays such as that illustrated in FIG. 1. Light source36 illuminates the screen 38. FIG. 2 illustrates the magnificationsystem where a reflective display 42 is used.

[0029] The first stage magnification process is an optical systemdesigned to simply enlarge the image. The first stage magnificationprocess may take the form of a single lens such as a simple asphericalconvex lens or it could take the form of a series of lenses. For thesake of simplicity, the first stage magnification optic will be referredto as an effective lens 32. It will be understood that effective lensrefers to the one or more lenses constituting the first stage. Theeffective lens 32 projects a magnified, collimated image onward to thesecond stage. The effective lens enlarges the projection enough so thatwhen the beam passes through the TIR lens, it fills the viewer's fieldof vision. Light also leaves the first stage at a predetermined anglechosen so that the beam will undergo total internal reflection beforeleaving the TIR lens 34.

[0030] In addition to magnifying the image, the effective lens 32 mayalso alter or correct the aspect ratio of the display. A lens ormultiple lenses can be designed so as to alter the size of the image inthe vertical and horizontal directions by different amounts. Forinstance, a 16:9 (width by height) image could be displayed on a 4:3screen so that it fills the entire screen. This would cause the image onthe screen to appear stretched and distorted. To eliminate thatdistortion the effective lens may be used to increase the width of theimage, decrease the height of the image, or increase both in such amanner that the width is increased by more than the height. This wouldallow for the use of all the pixel elements, eliminating the letterboxeffect and having a higher resolution image. To accomplish a greatermagnification to the width than to the height of an image, a single lenswould have to have a greater curvature along a vertical axis rather thanthe horizontal axis.

[0031] A mirror 50 is positioned between the first stage and the secondstage of the magnification process. The image projected from the firststage magnification process is reflected by the mirror into the TIR lens34. This kind of mirror is known as a folding mirror. The mirror mayalso have some corrective features because it is unlikely that the TIRlens will be flawless. The mirror can be designed to compensate forthese flaws.

[0032] The mirror 50 may also be integral with the TIR lens 34. See FIG.4. Incoming images pass through the base normal to the surface, so as toeliminate reflection. The mirror lies along surface 52 of the TIR lensand reflects the light onto surface 54 of the lens.

[0033] The TIR lens 34 is shown in FIG. 3. In general, the TIR lens iscomposed of a dielectric having a higher index of refraction than air.It has a small area inlet port 20, and a large area exit port 22. Thewall of the lens furthest from the viewer 24 is coating with areflective material. Various Aluminum compounds work well and arecommonly used. The inlet port 20 is located in front of the beam leavingthe first stage lens. The large area exit port 22 faces towards adownstream viewing area. The TIR is defined by two curved walls, thewalls having different radii of curvature.

[0034] A TIR lens has the property that the inner surface is reflectiveto light above a certain angle of incidence, θ_(i) with respect to thenormal to the surface at that point, and transmissive below θ_(i). FIG.8A shows a plot of transmission as a function of the angle of incidencefor light passing from air into the TIR lens. FIG. 8B shows transmissionas a function of the angle of incidence for light passing from withinthe first magnification optic into air. In FIG. 8B, the angle at whichtotal internal reflection occurs is shown for angles greater than theangle of total internal reflection, θ_(TIR). The angle of total internalreflection for a material can be calculated using the formulaθ_(TIR)=sin⁻¹(1/n), where n is the index of refraction of the material.The angle corresponding to θ_(i) on FIG. 8A and θ_(i1) on FIG. 8B can becalculated using Snell's Law. The relationship issin(θ_(i))=n*sin(θ_(i1)), where n is the index of refraction of thematerial forming the compound optical element. As a result, lightforming the magnified virtual image projected toward the inner surfaceof the exit port at an angle (θ_(MVI)) that is smaller than θ_(i1) istransmitted by the surface, while light from a source object projectedtoward the inner surface of the exit port at an angle (θ_(SO)) that isgreater than θ_(i1) is reflected back internally. The internallyreflected light then reflects off the back wall of the TIR, which iscoated with a reflective substance. The light then goes out the exitport. The increased distance that the light travels within the lensincreases the amount of refraction each light ray undergoes beforeexiting.

[0035] Because of the curvature of the walls of the TIR lens, andbecause different elements of the projected image are spread out over afinite distance, the different elements of the display image followdiverging paths and spread apart as they pass through the TIR lens.Because of this divergence, when the beams exit the TIR lens and travelonward towards the viewer's eyes, the viewer perceives a much largervirtual image. The exact magnification depends upon the exact shape ofthe lens and the angle of penetration of the beam from the first stagelens. The magnification process is illustrated best in FIG. 2 by theincreased spacing between the light rays exiting the exit port. Spatialplane 46 is where the viewer perceives the image to be. The angularspread of the light striking the viewer's eyeball 40 is what causes theviewer to perceive the larger more distant figure.

[0036] The relative distances separating the display, the first lens,and the TIR lens 34, depend upon the initial display size, the amount ofmagnification desired, the aspect ratio of the image, the relativedimensions of the display screen, the size and shape of the first lens,and the size and shape of the TIR lens.

[0037]FIG. 5 illustrates another embodiment of the two stage opticalsystem. In this embodiment a diffuser has been located in front of theTIR lens. This is sometimes necessary because the incident image willoften be highly collimated after passing through the first stagemagnification process. When a ray of light impinges on a diffuser, thediffuser emits a cone of rays corresponding to the incoming ray. Theangular spread of this cone is determined by the characteristics of theparticular diffuser. Because the spread of the cone is only dependentupon the particular diffuser used, each collimated ray of light producesa cone of rays with the same spread as every other collimated ray oflight. This allows that the viewer to see the entire image from avariety of angles. After passing through the diffuser, the refractioncaused by passage through the TIR lens increases the viewer's eyeboxeven further.

[0038] Another embodiment of the invention is shown in FIG. 6. In thisembodiment a prism 60 is placed in front of the TIR lens. Thecross-section of the prism is a triangle. In a preferred embodiment, thetriangle is a 30°-60°-90° triangle. It can actually have a variety ofdimensions. It just has to be shaped and positioned so that lightreflects off wall 66. For the particular embodiment using a 30°-60°-90°triangle, the hypotenuse of this triangle should be parallel with theinlet port 20 of the TIR lens. However this also does not have to be thecase. A right triangle is not required, so there may not even be ahypotenuse. Again the size, shape, and position of the prism areinterdependent parameters and if one is adjusted, the others need to beadjusted as well.

[0039] Light exiting the first stage of the magnification process,enters through a side of the prism. In the embodiment shown in FIG. 6,this is side 62, the side of the prism opposite the 30-degree angle.Incoming light internally reflects off of the front wall 64 of the prismand strikes the rear wall 66 of the prism 60. The rear wall 66 of theprism is coated with a projection diffusing coating. It is important tostress that this particular path for incoming light is not required. Ifthe prism had a different configuration the light path could and wouldbe different. The important element is that light traveling through theoptical system must reflect off the prism wall having the projectionscreen. Similar to the effect caused by the diffuser, for each light rayimpinging the rear wall 66, a cone of rays is emitted from the rear wall66 of the prism. This creates a greater range of directions in which theimage is being sent. The light exits through the front side of the prismand enters the TIR lens. The increased range of directions entering theTIR lens leads to a much larger eyebox for the viewer.

[0040] The coating used on the rear wall of the prism is preferablywhite, because white gives a flat response across the color spectrum.The angular spread of the reflected image is dependent upon whichcoating is used. Angular spreads of between 5° and 120° are possible.For the embodiment disclosed here, i.e., including a 30°-60°-90° prismpositioned as described, it was found that a 90° spread gave the largesteyebox without wasting any light rays of the reflected image. Use of aprism having a different size, shape, or position might requiredifferent angular spreads. If the angular spread is too large, the coneof rays for some elements of the image will extend past the edge of theTIR lens. Munsell White Reflectance Coating, a barium sulfate mixture,was found to work very well.

[0041] Yet another embodiment is shown in FIG. 7. This embodiment doesaway with a separate prism. This embodiment includes a reflective whiteprojection screen 68 incorporated into the TIR lens. Light entersthrough one end of the lens 70. It enters at an angle such that thelight passes through the lens and contacts the opposite wall of the TIRlens incorporating the projection screen 68. In response the projectionscreen emits a cone of rays. These light rays strike the inside surfaceof the large area exit port 74 at an angle greater than θ_(TIR) withrespect to a normal to the surface. The internally reflected light thenreflects off the back wall 72 of the TIR, which is coated with areflective aluminum compound. The light then goes out the exit port ofthe TIR lens. Again the cone of rays leads to a much larger eyebox forthe viewer.

[0042] While preferred embodiments of the invention have been shown anddescribed with particularity, it will be appreciated that variouschanges and modifications may suggest themselves to one having ordinaryskill in the art upon being apprised of the present invention. It isintended to encompass all such changes and modifications as fall withinthe scope and spirit of the appended claims.

What is claimed:
 1. A two-stage display system, comprising: (a) adisplay; (b) a first magnification optical system optically connected tothe display to magnify the display image; and (c) a second magnificationoptical system optically connected to the first stage optics system,where the second system includes a lens in which at least a portion ofthe incoming light is totally internally reflected.
 2. The displaysystem of claim 1 , wherein the display comprises one of aself-illuminating display, a back lit display, and a front lit display.3. The display system of claim 1 , where the first stage optical systemcomprises a double convex lens.
 4. The display system of claim 1 , wherethe first stage optical system includes an aspherical lens.
 5. Thedisplay system of claim 1 , where the first stage optical systemincludes a compound lens.
 6. The display system of claim 1 , where thefirst stage optical system includes multiple lenses.
 7. The displaysystem of claim 1 , where the display screen is less than 1.0″.
 8. Thedisplay system of claim 1 , where the display screen is less than 0.7″.9. The display system of claim 1 , where the display screen is less than0.5″.
 10. The display system of claim 1 , further comprising a diffuseroptically connected to the first and second optical systems to create alarger eyebox for the viewer.
 11. The display system of claim 1 , wherethe TIR lens incorporates a projection screen along one wall.
 12. Thedisplay system of claim 1 , further comprising a prism opticallyconnected between the first and second optical systems.
 13. The displaysystem of claim 12 , where one side of the prism is coated in aprojection diffusing substance.
 14. The display system of claim 13 ,where the projection diffusing substance is a barium sulfate compound.15. The display system of claim 1 , further comprising a mirroroptically connected to the first and second optical systems to fold thelight into the TIR lens.
 16. The display system of claim 15 , where themirror is attached to the TIR lens.
 17. A two-stage display system,comprising: (a) a display; (b) a first magnification optical systemoptically connected to the display to magnify the display image; (c) aprism optically connected to the first stage optical system; and (d) asecond magnification optical system optically connected to the prism.18. The display system of claim 17 , where one side of the prism iscoated in a projection diffusing substance.
 19. A two-stage displaysystem, comprising: (a) a display; (b) a first magnification opticalsystem optically connected to the display to magnify the display image;(c) a diffuser optically connected to the first optical system; and (d)a second magnification optical system optically connected to thediffuser.
 20. The display system of claim 19 , wherein the displaycomprises one of a self-illuminating display, a back lit display, and afront lit display.
 21. The display system of claim 19 , where the firststage optical system comprises a double convex lens.
 22. The displaysystem of claim 19 , where the first stage optical system includes anaspherical lens.
 23. The display system of claim 19 , where the firststage optical system includes a compound lens.
 24. The display system ofclaim 19 , where the first stage optical system includes multiplelenses.
 25. The display system of claim 19 , where the display screen isless than 0.5″.
 26. The display system of claim 19 , further comprisinga mirror optically connected to the first and second optical systems tofold the light into the TIR lens.